1. Field of the Invention
The present invention relates to a system for relaying and exchanging time division multiplex data, and particularly to an exchange system in which a route for achieving an in-slot transmission of signaling information and a route for achieving an out-slot transmission of signaling information are provided alternately, and pairs of transmission information composed of the time division multiplex data and the signaling information provided to control the transmission information for an exchange operation, are relayed to a remote terminal equipment via a plurality of stages of said routes.
2. Description of the Related Art
It is demanded to achieve transmission with high efficiency when transmission information, accompanied by signaling information, is transmitted to a remote terminal equipment through a plurality of stages of exchanges. To satisfy the need for highly, efficient transmission, a digital time division multiplex (TDM) method is widely employed. Particularly, on a transmission line between one exchange and another exchange, an Adaptive Differential Pulse Code Modulation (APCM) coding method of, for example, 32 Kbps, has been tried to achieve a highly efficient transmission of information. In this case, the signaling information is transmitted in such a manner that the signaling information is inserted in a part of a series of time slots which form the transmission information composed of the ADPCM data, which is the aforesaid in-slot transmission. In this case, the signaling information is separated from the time division multiplex data at the input and output sides of the exchange, namely the input and output sides of the time division multiplex data to be transmitted, so the aforesaid out-slot transmission is achieved. The reason for the above is that, since the signaling information contains data for setting up a line, dial data and the like, it is convenient for an exchange handling PCM data of, e.g., 64 Kbps, to separate the signaling information from the time division multiplex data.
In achieving the transmission of the signaling information in the in-slot transmission, it is desired to maintain synchronization between the signaling information appearing at the input and output sides of each exchange for relaying the information. If such synchronization is not maintained, as will be explained in detail hereinafter, the quality of the transmission information is deteriorated. The deterioration in quality is gradually accumulated every time the information is relayed by respective exchanges, whereby the greater the number of the relaying exchanges, the more the quality deteriorates.